In prior art systems, transponders are read by interrogation fields within Tunnel Reader Programmers (“TRPs”). Typically, the orientation and position of transponders as they enter the TRP are random and unknown. Accordingly, the TRP must switch its interrogation fields between orthogonal directions so that the transponders can be interrogated independently of their orientation. U.S. Pat. No. 5,258,766 provides an example of such a system.
There are a number of specific issues arising from the practical use of RFID transponders, in, say, luggage handling situations. These issues include the facts that, for example:                1. A mechanical means, such as a conveyor, moves luggage (and thereby transponders) through the TRP.        2. Several transponders may be attached to a single item.        
Where a mechanical means, such as a conveyor, moves items with attached transponders through the TRP the determination of the order of items on the conveyor is an essential requirement for allowing automated processing of the items. The determination of the order in which transponders enter a TRP is advantageous for determining the order of items on the conveyor. In prior art systems transponder order is normally inferred from the order in which they are identified. However, where multiple transponders are present the identification messages from these transponders may clash and the transponders may fail to be identified as they enter the TRP. When messages clash a further time interval will be required to correctly identify the transponders. During this time the transponders are moved further into the TRP by the conveyor. It is possible that subsequent transponder or transponders may enter the TRP before the first transponder is identified. It then becomes possible that one or more of the subsequent transponders may be identified before the first transponder. Consequently, the order of items may incorrectly be inferred from the order of transponder identification.
When the interrogation fields are switched, passive transponders power down within a relatively short time, at which point temporary data stored in volatile memory on board the transponder may be lost. Such data can include configuration information or temporary data stored in registers. In prior art systems configuration information or temporary settings required for transponder operation must be regenerated in the transponder after each switching of the interrogation field. This data can be read out of the transponder's memory or may have to be transmitted to the transponder by the TRP. This is undesirable because of the time delay involved. Moreover, in some cases, the data may no longer be available.
The above discussion is not to be taken as an admission of the extent of the common general knowledge in the field of the invention.